1. Field of the Invention
The present invention relates in general to the field of electronics, and more specifically to a switching power converter and control system.
2. Description of the Related Art
Power converters convert one source of input power into another source of power. For example, power converters convert alternating current (AC) voltage into direct current (DC) voltage or a first DC voltage into a second DC voltage. To maintain efficiency and meet international standards, power converters often incorporate power factor correction that minimizes phase and harmonic differences between input voltage and input current.
A large number and variety of applications utilize switching power converters to provide output power from a variety of input power sources. For example, high efficiency light sources, such as high frequency inverters for gas discharge lights (e.g. fluorescent lights) and light emitting diodes (LEDs), are powered utilizing DC voltages. However, power companies typically provide AC line voltages. Thus, one particular use of power converters is to convert AC line voltages into compatible, power factor corrected DC output voltages to provide power for high efficiency light sources.
Quadratic power converters for providing low voltage outputs from a wide range of input voltages were proposed by D. Maksimovic and S. Cuk in the article entitled “Switching Converter with Wide DC Conversion Range”, May 1989 Proceedings of the HFPC and also in the article entitled “Switching Converters with Wide DC Conversion Range” published in the Institute of Electrical and Electronic Engineer's (IEEE) Transactions on Power Electronics, January 1991. The topologies use a single switch to control cascaded buck and buck-boost stages. However, power factor correction generally cannot be provided using these topologies. In order to provide good power factor performance, an input stage receives an AC voltage and stores energy in a storage capacitor during half of each AC cycle. The storage capacitor provides energy for the other half of the cycle. A modified structure is needed for this performance to be possible.
FIG. 1a depicts a single switch power converter described in U.S. Pat. No. 6,781,351, entitled “AC/DC Cascaded Power Converters Having High DC Conversion Ratio and Improved AC Line Harmonics”, inventors Mednik et al., and filed on Oct. 28, 2002 (referred to herein as “Mednik”). The power converter described in Mednik can be used to convert AC line voltages into DC output voltages to power, for example, LEDs. Mednik describes a power supply 100 that combines an AC rectifier 102 with a first converter stage 104. An inductor L1 is connected in series with a blocking diode D1 to an input node 106 that receives a positive DC input voltage with respect to a common node 103. The voltage input signal Vx is a rectified AC voltage.
When switch Q1 is activated, inductor L1 is energized by causing switch Q1 to conduct and draw current from the input node 106 by alternately connecting switching input node 106 to common node 103. Blocking diode D1 prevents reverse flow of current to input node 106 when the voltage at input node 106 falls below the voltage at switching node 108, permitting only unidirectional current flow through inductor L1. Inductor L1 is energized from a zero current to a peak current proportional to the product of the on-state period of switch Q1 and the instantaneous voltage present at node 106. Simultaneously, capacitor C1 delivers stored energy to DC/DC converter stage 110. The DC/DC converter stage 110 provides constant voltage power to resistive load R_LOAD.
When switch Q1 is deactivated, current flows through a flyback diode D2 and blocking diode D1, causing the energy stored in inductor L1 to transfer to capacitor C1. Blocking diode D1 prevents reverse flow of current when the voltage at node 108 exceeds the instantaneous voltage at node 106, as a reverse current would otherwise occur once the inductor L1 current has reached zero. Diode D1 enables first power converter stage 104 to maintain a DC voltage at capacitor C1, while enforcing discontinuous inductor current mode in inductor L1. Capacitor C1 is selected to be sufficiently large in order to maintain a substantially DC voltage VC1 during operation of power supply 100.
Assuming that the duty ratio of switch Q1 is kept constant, an average input current into node 104 will be proportional to the instantaneous voltage at node 104, and power factor correction is achieved. Thus, inductor L1 draws input current only when switch Q1 conducts. The converter in Mednik achieves power factor correction by maintaining a constant relationship between the input voltage at node 106 and the current drawn through node 106 by inductor L1. However, the flyback diode D2 prevents input current from charging capacitor C1 when switch Q1 is non-conductive. Thus, the voltage of capacitor C1 never exceeds the peak input voltage at node 106. However, by preventing input current from charging capacitor C1 when switch Q1 is non-conductive, the control of switch Q1 is very simple. In exchange for simple control, the power supply 100 utilizes a relatively low voltage stored on capacitor C1. Storing a low voltage on capacitor C1 requires a relatively larger capacitor so that sufficient energy is stored by capacitor C1. Additionally, current is drawn from the input only during the ‘on’ time of the switch Q1, increasing the root mean square (RMS) current in the switch Q1, and requiring additional diodes, as shown in FIG. 4 of Mednik.
Switch state controller 112 controls the activation and deactivation, i.e. the conductivity, of switch Q1. U.S. Pat. No. 6,940,733, entitled “Optimal Control of Wide Conversion Ratio Switching Converters”, inventors Schie et al., and filed on Aug. 22, 2003 (referred to herein as “Schie”) describes an exemplary switch state controller 112. The switch state controller of Schie is used for producing a pulse train. The switch state controller 112 is coupled to either an input or internal node 106 of power supply circuit 100 to receive feedback signal FB1, i.e. the voltage VC1 across capacitor C1, for controlling the on-time (i.e. the pulse width) of each pulse in the pulse train. A frequency of the pulse train is controlled by a feedback signal FB2 coupled from an output characteristic of the power supply circuit 100.
FIG. 1b depicts a switching power converter 150 with load voltage monitoring as described in an article “Automatic Current Shaper with Fast Output Regulation and Soft-Switching” by Milivoje Brkovic and Slobodan Cuk, Telecommunications Energy Conference, 1993. INTELEC '93. 15th International, Sep. 27-30, 1993, Vol. 1, pages 379-386, ISBN: 0-7803-1842-0 (referred to herein as the “Brkovic Article”). The switching power converter 150 converts an input, time-varying voltage Vin into a DC load voltage VL The switching power converter 150 includes a switch Q1 that responds to a pulse width modulated control signal CS to alternately connect and disconnect inductor 154 and capacitor 156 to a common reference voltage −Vin. The switching power converter 150 energizes inductor 154 when switch Q1 conducts. When switch Q1 is non-conductive, inductor 154 provides stored current to capacitor 156. Switching power converter 150 operates in discontinuous inductor current mode, so inductor 154 is completely discharged prior to switch Q1 becoming conductive. Diode 152 prevents reverse current flow into the +Vin terminal Capacitor 156 and inductor 158 provide a constant load current iload to load 160. Diode 164 prevents reverse current flow into inductor 158. Inductor 154, capacitor 156, and inductor 158 have respective values of L1, C1, and L2.
The switching power converter 150 includes a switch control circuit 162 to control the switching frequency of switch Q1. Switch control circuit 162 monitors the load voltage VL with respect to a reference voltage Vref. The switch control circuit 162 modulates the frequency of control signal CS in response to changes in the load voltage VL. The Brkovic Article indicates that the switching power converter 150 requires a modulation index (MI) of greater than 2 to obtain a total harmonic distortion of less than 13%. The MI is defined by the Brkovic Article as the capacitor voltage VC divided by the peak input voltage Vin—peak, i.e. VC/Vin-peak. When the switching power converter 150 is operated in discontinuous inductor current mode and at a constant duty cycle ratio of control signal CS, the switching power converter 150 is designed so that the input current iin becomes automatically proportional to the line voltage Vin to achieve power factor correction.